Sealing devices such as sealing rings and sealing strips are used in many applications to provide gas- and liquid-tight seals between mating surfaces. In applications such as seals between the mating surfaces of a lid and an opening, at least one of the mating surfaces is usually provided with a positioning groove, shoulder or flat region intended to receive a sealing device in the form of a loop or ring or strip of resilient sealing material. In the specific case of a single positioning groove or shoulder the sealing device is dimensioned so that before the mating surfaces are brought together it can extend above the sides of the positioning groove, and after the two mating surfaces are correctly positioned and brought together it is in contact with both mating surfaces and exerts a sealing force against them. As resilient sealing devices can only change shape but normally cannot change volume by any significant amount the dimensions of the sealing device has to be carefully matched to the dimensions of the groove and obviously the volume of the sealing device has to be smaller than the volume of the groove it is intended to be used in. In the case of two positioning grooves where one groove is provided in each of the opposing mating surfaces, the total height of the sealing device is normally greater than the combined depths of the two positioning grooves. Current sealing devices are usually made of elastomer material and in order to function as seals have to flexible and preferably soft (i.e. they are made of material with a low Young's modulus). Fitting large sealing devices which are highly flexible into grooves is difficult to automate as the sealing devices have the rigidity of well-cooked spaghetti and thus tend to droop out of shape. This means that they often need to be fitted manually.
It can be easy to fit and retain resilient sealing devices (particularly when the devices have a circumference less than about 30 cm) in grooves with only concave bends (e.g. circular grooves) while the components are being handled or transported, but in more complex shaped grooves which include convex bends it can be difficult to correctly position and retain the sealing device in the groove as its flexibility allows it to take up unwanted orientations (for example it will tend to contact the inner radius of a bend instead of laying in the middle of the groove) and/or to stretch unevenly (thus, for example, becoming undesirably thinner in the region of a bend) or even to fall out of the groove. In order to overcome these problems, a sealing device 1 can be provided with regularly spaced lateral projections 3 which can grip the wall 5 of the groove 7 formed in the mating surface 9 of a component 11 and thereby hold the device in place as is shown in FIGS. 1a) and 1b). Such lateral projections increase the cost of the sealing devices but do not solve the problem of how to easily and reliably fit the flexible sealing devices in the grooves as such projections do not add significantly to the rigidity of the sealing device.
In order to overcome the above mentioned problems it is known to provide seals in the form of gaskets made from substrates in the form of sheets of thin rigid material, for example a metal or composite material, provided with resilient material, for example a rubber, on one or more faces. Usually the resilient material is provided on two opposing faces. An example of such a gasket for use as a cylinder head seal in internal combustion engines is known from EP690252. It is taught in this patent that a gasket can be formed by cutting the desired shape out of a sheet of composite material comprising a reinforcing electro-galvanised metal sheet positioned between two layers of elastomer. Cutting takes place between two rotating blades. A problem which arises when cutting sheet metal in this method is that it is a slow process, limited by the speed of rotation of the blades, which means that the cost of the final gasket is high.
GB1272523 relates to method for producing rigid gaskets or sealing rings comprising a metal stock layer with either one or two gasket stock layers. The metal stock layer is normally a flat plate of metal which has two parallel major surfaces which are separated by the thickness of the plate. The gasket stock layer material comprises a sheet of rubber with two major surfaces separated by the thickness of the rubber. The patent describes two basic approaches for fabricating gaskets. The first method is to join the metal and gasket material together with a major surfaces of the metal plate and rubber sheets in contact, and to die cut them as a unit. In die cutting the component which is to be cut out is placed between a punch and a die. The desired shape of the component is achieved by the edges of the punch and die passing each other. Usually the die has an opening which is larger than the punch to allow smooth entry of the punch into it. The second method is to die cut the metal and the gasket material separately and then join them together. A problem with the first method is that it not possible to accurately control the width or diameter of the resulting gasket. This is because die cutting leaves broken surfaces as the cutting proceeds through the material and tends to cause the composite material to become bowed or warped, i.e. the major surfaces of the gasket are no longer in straight parallel planes. This can be seen in FIGS. 2-4 which shows simple die cutting as known in the prior art. FIG. 2a) shows schematically a punch 21 and a die 23 used to cut thorough a piece of composite material 25—the parent stock 22—in order to form a part 24. Composite material 25 is made out of a substrate 27 of rigid sheet material coated on both major sides with respectively an upper sheet 29 and a lower sheet 31 of resilient material. Punch 21 has a cutting edge 33 which can move vertically as shown by an arrow. Die 23 has a cutting edge 35 which is offset a distance d from the path that punch cutting edge 33 follows. Distance d is the cutting clearance. Cutting of composite material 25 is achieved by moving the punch 21 to the position shown by dotted lines. In this position the punch cutting edge 33 has passed parallel to, but at distance d from, die cutting perimeter 35 thereby separating the part 24 from the parent stock 22.
FIG. 2b) shows schematically a typical cross-section through the part 24 of composite material which has been cut by such a punch and die. The cut edge 30 of upper layer 29 of resilient material has a convex cross-section caused by some of the resilient material being stretched and dragged down by the flat face of the punch surrounding the punch cutting edge before fracturing. The cut edge 32 of lower layer has a concave cross-section. The cut edge of substrate 27 of rigid material is bend downwards due to it being plastically deformed before being penetrated and fracturing. As can be seen in the enlarged view of FIG. 2b), the cut edge 38 of the rigid substrate 27 exhibits an upper region 37 of edge rollover of height “r” where plastic deformation took place, a shiny smooth or burnished middle region 39 of height “p” where penetration took place, a rougher lower region 41 of height “f” when the material fractured and it may also have a burr edge 43 of height “e”.
FIG. 3a) shows schematically a punch 21′ and a die 23′ used to cut a strip through a piece of composite material 25. Punch 21 has two cutting edges 33′, 34′ which together can move vertically as shown by arrows. Die 23 has a cutting perimeter 35′ which is offset the cutting clearance distance d from the path that each punch cutting edge 33′, 34′ follow. Cutting of composite material 25 is achieved by moving punch to the position shown by dotted lines. In this position the punch cutting edges 33′, 34′ have passed parallel to, but at distance d from, die cutting edge 35′.
FIG. 3b) shows schematically a typical cross-section through a piece of composite material which has been cut by such a punch and die. The resulting disk 36′ is not flat but is domed. The cut edge 30′ of upper layer 29 of resilient material has a convex cross-section caused by some of the resilient material being stretched and dragged down by the punch cutting edge before fracturing, the cut edge 38 of substrate 27 of rigid material is bend downwards due to it being plastically deformed before being penetrated and fracturing and the cut edge 32′ of lower layer 31 has a concave cross-section due to it being ripped by the descending cutting edge. The cut edges 38′, 40′ of the substrate each exhibit an upper region of rollover where plastic deformation took place, a shiny smooth burnished middle region where penetration took place, a rougher lower region when the material fractured and they may also have a burr edge.
FIG. 4a) shows schematically a punch 21″ and a die 23″ used to cut through a piece of composite material 25 to form an annular piece such as a washer. Punch 21″ has outer diameter cutting edge 33″ and inner diameter cutting edge 34″ which together can move vertically as shown by arrows. Die 23 has an outer cutting edge 35a″ and an inner cutting edge 35b″ which each are offset the cutting clearance distance d from the path that the punch cutting edge 33″ respectively, 34′ follow. Cutting of composite material 25 is achieved by moving the punch to the position shown by dotted lines. In this position the punch cutting edges 33″, 34″ have passed parallel to, but at distance d from, die cutting edges 35a″ and 35b″. 
FIG. 4b) shows schematically a typical cross-section through a piece of composite material which has been cut by such a punch and die. The resulting washer 36″ is not flat but is bowed. The cut edges 30″ of upper layer 29 of resilient material has a concave cross-section caused by some of the resilient material being stretched and dragged down by the punch cutting edge before fracturing, while the substrate 27 of rigid material is bend downwards due to it being plastically deformed before being penetrated and fracturing and the cut edges 32″ of lower layer 31 of resilient material has a concave cross-section due to it been ripped by the descending cutting edge. The cut edges 38″, 40″ of the substrate each exhibit an upper region of edge rollover where plastic deformation took place, a shiny smooth middle region where penetration took place, a rougher lower region where the substrate material fractured and they may also have a burr edge.
Bowed gaskets formed as a unit by means of die stamping are unacceptable to the modern manufacturing industry in which manufacturing tolerances are continuously becoming tighter. The second method described in GB1272523 is expensive as each component is cut separately and then has to be assembled into a unit. This requires alignment and adhesion steps which are time-consuming and expensive. Additionally such rigid gaskets and sealing devices are more expensive to store and transport compared to flexible sealing devices due to the large areas enclosed within the perimeter of the gasket.